Electrical musical instrument



Jan. 31, 1950 T. H. LONG 2,496,224

ELECTRICAL MUSICAL INSTRUMENT Filed Aug. 31, 1945 Patented Jan. 31, 1950 ELECTRICAL MUSICAL INSTRUMENT Thomas H. Long, Elkhart, Ind., assignor to C. G. Conn, Ltd., Elkhart, Ind, a corporation of Indiana Application August 31, 1946, Serial No. 694,382

7 Claims.

This invention relates to electrical musical instruments and more particularly to electric oscillators adapted for use in instruments of the electric organ type.

In electrical instruments it has been customary to provide a tremolo eifect by periodically altering the frequency of the tone. In instruments of the type employing electric oscillators as tone generators this has generally been accomplished by using an oscillator capable of frequency variations and periodically varying its frequency in response to voltage supplied by a low frequency tremolo generator. The difiiculty of this construction is that any oscillator capable of frequency variations in response to changes in bias on the amplifier tubetends to be unstable and to change its frequency in response to any change in tube characteristics such as that caused by normal aging. Furthermore this arrangement produces a tremolo effect in all tones whereas ordinarily it is desired only in those tones which are rich in harmonics such as string tones and not in relatively pure tones such as flute tones.

It is accordingly an object of the invention to provide an oscillator Whose frequency is sub stantially independent of variations in tube characteristics.

It is another object of the invention to provide an electrical musical instrument in which a tremolo effect may be produced without substantially affecting the frequency of the tones.

Another object is to produce an instrument in which a tremolo effect is produced by periodically varying the quality or wave form of the tones.

Still another object is to provide an instrument in which a tremolo effect is produced only in those tones which are relatively rich in harmonics.

The above and other objects and advantages of the invention will be more readily apparent from the following description when read in connection with the accompanying drawing, in which the single figure is a diagrammatic view of an oscillator for an electrical musical instrument embodying the invention.

The oscillator circuit as shown is substantially similar to that more particularly described and claimed in the patent to McKellip et al. No. 2,340,002 for producing tone frequency currents of two different characters from a single oscillator circuit. The circuit, as shown, comprises an electron discharge tube Ill having a cathode H, a control grid l2 and an anode or plate l3. The anode is connected to a resonant tank circuit including an inductive winding l4 having a capacitor l5 connected across it. The winding l4 comprises in effect a split winding, and the B supply for the plate is supplied to the mid point thereof through a lead IS. The tank circuit is connected to the control grid l2 through a blocking condenser I1 and a stabilizing resistor l8. In the construction shown the resistor l8 together with other circuit elements to be described serves-the purpose of stabilizing the frequency of theoscillator so that it will not be subject to any substantial frequency variations.

Tone frequency currents are derived from the tank circuit through a transformer indicated generally at 49 for which the winding I4 comprises the primary. The transformer has a secondary winding 2| which is connected across a, potentiometer resistance 22 so that the amplitude'of the output can be adjusted. The output is transmitted through wires 23 one of which variably engages the resistance 22 to a suitable signal reproducing system as shown, for example, in said Patent No. 2,340,002. The tone frequency currents derived from the tank circuit through the transformer H) are substantially pure sine wave currents to produce a flute tone.

The grid and cathode are connected by a circuit including a pair of resistors 24 and 25 in series with the resistor 18 and with a source of Cbias 26. The cathode is connected to a wiper 21 movable over an output resistor 28 which may be connected through a lead 29 to the tone reproducing means. The lower end of the resistor 28 is grounded at 3!, as shown. As explained in the McKellip patent, the tone frequency currents derived from the resistor 28 will be rich in harmonies and will produce tones similar to string tones. It will be understood that the several tone currents can be mixed as desiredto obtain different tone effects.

, The bias impressed on the control grid l2 by.

the C bias source 26 is sufficient to bias the tube to cut off so that it will normally not conduct.

For keying the circuit to start it oscillating when desired, a key switch 32 is provided to connect the point between resistors 24 and 25 to ground at 33. When the switch 32 is closed, the bias source will be shunted to remove the bias from the control grid so that the tube will conduct and the circuit will start to oscillate.

To provide a tremolo effect a low frequency tremolo generator 34 which may be a low frequency oscillator is provided. One side of the generator is connected to ground at 33 and the other side is adapted to be connected in series with the keying switch 32 by a switch 35. With the switch 35 in the full line position shown,

the tremolo generator is out of the circuit and no tremolo effect will be produced. When the switch 35 is moved to its dotted line position, the tremolo generator will be in effect connected across the resistor 25 so that it will be efiective.

In the circuit as illustrated I have found that by selection of a proper value for the resistance l8, the circuit will be highly stabilized and will be made substantially independent of variations in tube characteristics or of changes in bias on the tube. In fact if the resistance I8 is made too large the circuit can be made to reverse itsnormal direction of frequency change for a given change in tube characteristics. In one example using a 12AH'? tube with a plate voltage of about 105 volts and a frequency between 60 and 120 C. P. S. I have found that the resistor l8 should be from about 470,000 to 820,000 ohms. For lower frequencies the value of the resistor should be somewhat higher. While the effect of changes in tube characteristics can be completely eliminated by careful selection. of the resistor I8, I have found that the value of this resistor is not extremely critical and that an approximately correct value of the resistor will eliminate about 98% of the effect of tube characteristic changes.

Preferably the tremolo generator is so designed as to produce a voltage fluctuation on the order of not. to exceed about five volts which is insufficient to effect the frequency of the oscillator. I have found however that when the tremolo voltage is impressed on. the circuit in the manner shown, it will produce a periodic-variation in' the quality or the wave form of the tone frequency currents derived from the grid cathode circuit through the resistor 28. This periodic change in quality produces an effect on the listener which is very pleasing and which is somewhat similar to the effect produced by the usual frequency changes accompanying a tremolo. While the generator will vary the quality of the tone frequency currents derived from the grid cathode circuit, it'will not appreciably effect the currents. derived from the tank circuit. This is highly desirable since it is. ordinarily desired only to have a tremolo in those tones which are rich in harmonics such' as the string tones or mixed' tones and not in the relatively pure tones such as. flute tones.

While one embodiment of the invention has; been shown and described in detail herein, it will be understood that this is illustrative only and not intended as a definition of the scope of the. invention, reference being had for this purposeto the appended claims.

What is claimed is:

1. In an electrical musical instrument, an oscillator circuit including an electron discharge tube having acathode, an anode: and a control grid, means in the circuit to stabilize it so that the oscillator will oscillate at a substantially constant. frequency; means to supply a bias. to the control grid to bias the tube. to cut-011a shunt circuit around the biasing means including, a switch adapted to be closed to start the oscillator, aloyv frequency generator, and means to connect the generator in the. shunt circuit.

2. In an electrical musical instrument;.an oscillator circuit including an electron discharge tube having a cathode, an anode, and a control grid, a circuitv connecting the cathode and the control grictmeans for deriving tone frequency current fromthe circuit, a low frequency tremolo genera- 0 Number tor, and means for connecting the generator in the cathode-grid circuit to change the wave form of the derived tone frequency current.

3. In an electrical musical instrument, an oscillator circuit including an electron discharge tube having a cathode, an anode, and a control grid, a circuit connecting the anode and grid, means for deriving tone frequency current from said circuit, a circuit connecting the grid and cathode, means for deriving tone frequency current of a different character from the last named circuit, a low frequency generator, and means to connect the generator in the grid-cathode circuit to change the wave form of the tone frequency current derived from the grid-cathode circuit while maintaining substantially constant the frequency of oscillation of the oscillator circuit.

4. In an electrical musical instrument, an oscillator circuit including an electron discharge tube having a cathode,an anode, and a control grid, a circuit connecting the: anode and grid and including a resistor tov stabilize the frequency of oscillation of the tube, a. grid-cathode circuit including a pair of resistors in series, means. to derive tone frequency currents. from. the grid-cathode circuit, a low frequency tremolo generator, and means to connect the. generator across one of said pair of resistors.

5. In an electrical musical instrument, an 05- cillator circuit including an electron discharge tube having a cathode, an anode, and a control grid, av tuned circuit connected to the anode, means including a series resistor connecting the tuned circuit to the control grid, means-to derive tone frequency' currents from the tuned circuit, a circuit including two resistors in series connecting the grid and cathode, means to derive tone frequency currents. from the last named circuit, a low frequency tremolo generator, and means to connect the. generator between the cathode and a point between thev two resistors to vary the wave form of the last named tone frequency currents without substantially affecting. their frequency.

6. In an electrical musical instrument, an 05- cillat-or circuit including an electron discharge tube having a cathode, an anode, andv a control grid, a circuit connecting the cathode and the control grid, a resistor in series. in the circuit, means for deriving tone frequency: current from the circuit, a low frequency tremolo generator, and means to connect the tremolov generator across said resistor.

'7. In an oscillator, a three: electrode electron dischargedevicehaving a cathode, an anode, and a control grid, a tank circuit operably: connected to-the anode and to the'control grid, a; frequency stabilizing resistance in series to the: grid, a circuit including a resistance in series connecting the cathode through said first named resistance to the control grid, and means for deriving a signal from the last named: resistor.

THOMASH. LONG.

REFERENCES CITED The following references are of' record in the file of'this patent:

UNITED. STATES PATENTS.

Name Date Urtel July 19} 1932 Kock' Eng. 30'; I938 McKellip Jan; 25. 1944 

